""168 NERVE TISSUE IN THE LOWEST ORGANISM. 



instances, nerve action, which is dependent upon 

 changes in the bioplasm, never ceases for a single 

 moment from birth to death. The active phenomena of 

 the neiwous system are entirely dne to vilal actions, and 

 the matter which is concerned in these vital changes 

 is invariably the bioplasm We shall therefore have 

 to consider the general structure and mode of growth 

 of nerve tissue, and especially the relation -which its 

 bioplasm bears to the formed material of nerve- fibres 

 and cells. The mere anatomical question has, indeed, 

 a most important bearing, for, until it is decided, we 

 •cannot hope to carry on investigations with any 

 great chance of success into the real nature of nervous 

 actions. Various. opinions have been advanced con- 

 cerning the structure and arrangement of nerves in 

 the lowest animals, and the fanciful suggestion that 

 nerve matter may be in solution or in a molecular state, 

 diffused through the general tissues of the body, has 

 scarcely yet been abandoned. On the other hand, it has 

 been affirmed that no fibre oughtto be regarded as nerve 

 which does not exhibit the dark-bordered character. 

 This strange dictum involves the acceptance of the 

 doctrine that nerves pass into, and are continuous 

 with, other tissues of the body, and act by reason 

 of this continuity of material substance. But I hope 

 to convince you that we may now discard both these 

 extreme doctrines, because we are able to define, and 

 with considerable accuracy, what is essential to nerve 

 structure. 



227. ;Nerve tissue in the lowest oi'&ranisms. — It might 

 be supposed that we should be able to form a correct 

 idea of the essential structure of a nervous apparatus 

 if we appealed to some of the lowest organisms in 

 which the existence of nerve tissue might be fairly 

 assiimed. In them it would be supposed that we should 

 meet with a neiwous system in a very simple con- 

 dition. But it unfortunately happens that in these 

 lower forms of life the nerve-fibres are so very deli- 



